Typically a tractor drawbar extends rearward from the tractor and has a vertical drawbar pin hole for attachment of an implement about a substantially vertical yaw axis so that the implement can pivot with respect to the tractor during turns. In a typical implement hitch it is also desirable to allow the implement to move independently of the tractor so that the implement can follow the ground. It is desired that the implement can pivot about a roll axis that is substantially parallel to the drawbar, and also about a pitch axis that is substantially perpendicular to the drawbar. These pitch and roll axes can be provided by a ball and socket connection however the degree of movement allowed is often not sufficient for the particular application. The axes can also be provided by an arrangement of pins through corresponding holes in brackets or the like, such as the hitch apparatus disclosed in U.S. Pat. No. 6,203,049 to Gibson. In lighter duty applications often a loose connection is simply provided at the draw bar attachment by using a small pin in a larger hole.
Implements such as rotary mowers also are typically powered by a drive shaft attached through a universal joint at the front end to the power take off (pto) of the tractor, and attached at the rear end through another universal joint to the implement drive line. In order to maintain constant velocity of the drive shaft, and thus avoid excessive vibration during turns, the vertical yaw axis, about which the implement and tractor pivot with respect to each other, should be located in the middle of the drive shaft, equidistant from each universal joint.
Such a configuration is disclosed in U.S. Pat. No. 3,557,892 to Burrough who uses a drawbar extension to move the yaw axis to the middle of the drive shaft. Similarly U.S. Pat. No. 4,195,704 to Bass et al. uses a drawbar extension for the same purpose. In U.S. Pat. No. 4,403,670 to Sammarco, the effective length of the drawbar itself is adjusted to accomplish the purpose of locating the yaw axis in the middle of the drive shaft.
The downward force exerted by an implement on the drawbar of a towing vehicle can be significant. For example, wing-type rotary mowers typically have wheels at the rear end thereof, and are supported at the front end by the drawbar of a towing tractor. Tractors with sufficient power to otherwise operate such mowers often do not have a drawbar with sufficient strength to bear the weight of the front hitch end of the mower.
Typically the tractor drawbar extends rearward from the tractor. The drawbar cantilevers out from the tractor, and when excessive weight is placed on the rear end of the drawbar, it can bend. The weight of the implement hitch exerts a bending force on the drawbar that is substantially equal to the weight of the implement hitch times the distance between the drawbar support on the tractor and the implement hitch.
Hitches in the prior art are also known where a mower is attached to the to the drawbar by a conventional drawbar pin in the drawbar hole, and includes a mechanism whereby the pitch axis is located forward of the drawbar hole. The weight of the implement is exerted downward at a fore and aft location dictated by the pitch axis which is perpendicular to the drawbar, and so the bending forces are reduced compared to conventional attachment where the pitch axis is located substantially at the drawbar hole, or rearward of the drawbar hole. In the Bush Hog™ hitch, the pitch axis is located above the roll axis, and the yaw axis is located at the drawbar hole.
In the prior art mechanisms for adjusting the position of the yaw axis to the middle of the drive shaft the distance between the drawbar support on the tractor and the implement hitch is increased, thereby increasing the bending forces on the drawbar. The weight of the implement is exerted substantially at the yaw axis. Especially with larger rotary mowers, the increased bending forces can be problematic, requiring a heavier drawbar than is normally provided on a tractor with sufficient power to otherwise operate the mower satisfactorily.